1. Field of Invention
The present invention relates to a data communications system, and in particular to the control of access by users to copies of digitally encoded data. It is applicable, for example, to the control of data multicast on the internet.
2. Description of Related Art
Multicasting routing techniques have been developed to allow multiple copies of a data item to be distributed efficiently to a large number of end users. However, for such techniques to be exploited commercially, it necessary to control selectively access by users to the data. For example, in an application in which selected stock market prices are multicast via the internet to subscribers, it is necessary to ensure that the data is accessed only by users who have paid the relevant subscription. This might be achieved by encrypting the data and only releasing the relevant key to the user in return for the subscription payment. However, whenever one user's subscription expires, after a fixed length of time or after a predetermined quantity of data has been received, it would be necessary to change the key for all the users, in order to exclude the one user. In such a situation, the traffic associated with key distribution becomes a significant operating overhead, and may even exceed the traffic for the data itself.